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Red‐ and Blue‐Shifted Hydrogen Bonds in the Cis–Trans Noncyclic Formic Acid Dimer
Author(s) -
Zhou PanPan,
Qiu WenYuan
Publication year - 2009
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.200800870
Subject(s) - dimer , chemistry , formic acid , hydrogen bond , photochemistry , stereochemistry , organic chemistry , molecule
The cis–trans noncyclic formic acid dimer was studied by means of MP2 method with 6‐31G(d,p), 6‐31+G(d,p) and 6‐311+G(d,p) basis sets. It exhibits simultaneously red‐shifted OH⋅⋅⋅O and blue‐shifted CH⋅⋅⋅O hydrogen bonds. AIM and NBO analyses are performed at the MP2/6‐31+G(d,p) level to explore their properties and origins. AIM analysis provides the evidence that the OH bond becomes weaker and the CH bond becomes stronger upon the hydrogen bond formations. Intermolecular and intramolecular hyperconjugations have important influence on the electron densities in the XH (X=O, C) σ bonding orbital and its σ* antibonding orbital. The electron densities in the two orbitals are closely connected with the XH (X=O, C) bond length, and they are used to quantitatively estimate the bond length variation. The larger amount of charge transfer in the red‐shifted OH⋅⋅⋅O hydrogen bond is due to its favorable H⋅⋅⋅O electron channel, whereas the H⋅⋅⋅O electron channel in the blue‐shifted CH⋅⋅⋅O hydrogen bond is weaker. Structural reorganization effects shorten the CH bond by approximately 30 % when compared to the CH bond contraction upon the dimerization. Strikingly, it leads to a small elongation and a slight red shift of the OH bond. Both rehybridization and repolarization result in the XH (X=O, C) bond contraction, but their effects on the OH bond do not hold a dominant position. The hydrogen‐bonding processes go through the electrostatic attractions, van der Waals interactions, charge‐transfer interactions, hydrogen‐bonding interactions and electrostatic repulsions. Electrostatic attractions are of great importance on the origin of the red‐shifted OH⋅⋅⋅O hydrogen bond, especially the strong H δ+ ⋅⋅⋅O δ− attraction. For the blue‐shifted CH⋅⋅⋅O hydrogen bond, the considerable nucleus–nucleus repulsion between H and O atoms caused by the strong electrostatic attraction between C and O atoms is a possible reason for the CH bond contraction and its blue shift.